1. Field of the Invention
The present invention relates generally to a developing unit used in copiers, facsimile machines, printers, or other image forming apparatuses, and more specifically, to a developing unit using a two-component developer containing toner and magnetic carrier, a process cartridge using the developing unit, and an image forming method and apparatus using the developing unit.
2. Description of the Background
An image forming apparatus used as a copier, facsimile machine, printer, or multi-functional device thereof may have a developing unit to develop an image with two-component developer containing toner and carrier.
Such a conventional developing unit has a configuration as illustrated in FIG. 1, for example. In FIG. 1, a conventional developing unit 104 has two developer compartments, that is, a supply recovery compartment 402 including a supply screw 401 and an agitation compartment 10 including an agitation screw 11. The supply recovery compartment 402 and the agitation compartment 10 transport developer in opposite directions to circulate the developer in the conventional developing unit 104.
In the conventional developing unit 104, the developer is supplied from the supply recovery compartment 402 to a surface of a developing roller 5 and is used for development in a development area in which the developing roller 5 faces a photoconductor, not illustrated. After passing through the development area, the developer is recovered from the developing roller 5 to the supply recovery compartment 402.
In the conventional developing unit 104, a single compartment, that is, the supply recovery compartment 402 performs both functions of supplying developer to the developing roller 5 and recovering the developer passed through the development area. Consequently, on a downstream side of the supply recovery compartment 402 in its developer transport direction, the concentration of toner in the developer to be supplied to the developing roller 5 may decrease, thereby resulting in failures such as a reduction in image density during a development process.
Accordingly, certain conventional developing units have a supply transport member that supplies and transports developer to the developing roller and a separate recovery transport member that recovers and transports the developer passed through the development area in separate developer compartments.
FIG. 2 illustrates one type of conventional developing unit 204 having such configuration adjacent a photoconductor 1.
The conventional developing unit 204 separately has a supply compartment 9 to supply developer to a developing roller 5 and a recovery compartment 7 to recover the developer passed through a development area. As illustrated in FIG. 2, the conventional developing unit 204 has a separation member 133 separating the supply compartment 9 and the recovery compartment 7, which recovery compartment includes a recovery screw 6. The separation member 133 has an end portion 133a facing the developing roller 5. The supply compartment 9 and the recovery compartment 7 each are disposed to face the developing roller 5. The supply compartment 9 is disposed above the recovery compartment 7 behind the separation member 133. FIG. 2 also depicts a gap Gp, between the developing roller 5 and a supply screw 8.
In the conventional developing unit 204 thus configured, the developer passed through the development area is transported to the recovery compartment 7 and thus is not mixed into the supply compartment 9. Such configuration eliminates a change in toner concentration of the developer transported to the supply compartment 9, thereby allowing the toner concentration of developer supplied to the developing roller 5 to be maintained substantially constant.
FIG. 3 illustrates another type of conventional developing unit 304.
The conventional developing unit 304 has a supply compartment 9 to supply developer to a developing roller 5 and a separate recovery compartment 7 to recover the developer passed through a development area. The conventional developing unit 304 also has an agitation compartment 10 that agitates excess developer, transported to a downstream end portion of the supply compartment 9, and recovered developer, transported to a downstream end portion of the recovery compartment 7, which includes a recover screw 6, into agitated developer and simultaneously transports the agitated developer in a direction opposite to the developer transport direction of the supply compartment 9.
As illustrated in FIG. 3, the conventional developing unit 304 also has a separation member 133 separating the supply compartment 9 (and associated supply screw 8) and the recovery compartment 7 (and associated agitation screw 11). The supply compartment 9 and the recovery compartment 7 are disposed to face the developing roller 5. The separation member 133 has an end portion 133a facing the developing roller 5. The supply compartment 9 is disposed above the recovery compartment 7 behind the separation member 133.
As is the case with the conventional developing unit 204 described above, in the conventional developing unit 304 thus configured, the developer passed through the development area is transported to the recovery compartment 7 and thus is not mixed into the supply compartment 9. Such configuration eliminates a change in toner concentration of the developer transported to the supply compartment 9, thereby allowing the toner concentration of developer supplied to the developing roller 5 to be maintained substantially constant.
As illustrated in FIGS. 2 and 3, in each of the conventional developing units 204 and 304, the supply compartment 9 is disposed above the recovery compartment 7 behind the separation member 133, which has the end portion 133a facing the developing roller 5. However, such configuration may result in failures depending on the size of a gap between the end portion 133a and the developing roller 5.
For example, if the gap between the end portion 133a and the developing roller 5 is too great, a portion of developer in the supply compartment 9 may not appropriately be supplied to the developing roller 5 and may drop through the gap into the recovery compartment 7.
Such dropping of developer into the recovery compartment 7 may result in failures as follows. Specifically, when the developer dropping through the gap reaches the recovery compartment 7 in addition to the developer supplied to the developing roller 5 from the supply compartment 9, the amount of developer consumed from the supply compartment 9 may increase. Consequently, developer may run short in a downstream portion of the supply compartment 9 in its developer transport direction, thereby resulting in shortage of developer to be supplied to the developing roller 5.
Further, such dropping of developer into the recovery compartment 7 may increase the amount of developer contained in the recovery compartment 7. As a result, the thickness of developer may increase on the downstream side of the recovery compartment 7 in its developer transport direction, thereby preventing the developer on the surface of the developing roller 5 from appropriately dropping to the recovery compartment 7 after a developing process.
Further, such preventing may result in a so-called “taking-around” phenomenon, in which, after passing through the development area, the developer remains on the surface of the developing roller 5, passes through a facing area in which the developing roller 5 faces the supply compartment 9, and reaches the development area again. Such taken-around developer may repeatedly pass through a doctor gap 5a and thus be degraded by friction, thereby resulting in uneven degradation of developer.
Further, such taken-around developer may be heated by friction at the doctor gap 5a. Accordingly, if the taking-around phenomenon occurs when using a toner having a low melting point, the toner is repeatedly heated at the doctor gap 5a before being cooled down, thereby causing the toner to be melted while being taken around with each rotation of the developing roller 5. Such melting of toner in two-component developer carried on the developing roller 5 may result in such failures as, for example, fusion of the toner on the developing roller 5, or aggregation of toner particles.
Consequently, there is still a need for a developing unit, process cartridge, image forming method and apparatus capable of preventing failures that may be caused by developer dropping through a gap between a developer carrier and an end portion of a separation member.